This invention relates generally to time displacement of time varying signals particularly suited to compensate time base errors that occur in a signal reproduced from a medium which stores the time function of the signal. When reproducing signals stored in such a medium, ideally, the reproduction operation is carried out under conditions identical to those existing during the record operation for the purpose of avoiding or minimizing the introduction of unacceptable time base errors in the reproduced time varying signal. Unfortunately, however, general time base errors can occur from a variety of causes. For example, in a magnetic storage medium, signal timing errors are commonly caused by changes in medium dimensions due to environmental effects, such as stretching of magnetic tape due to tension produced by a tape transport, head tip medium penetration changes or differences in the relative head to medium record and reproduce velocities. Relative head to medium velocity errors ordinarily occur as a result of dimensional changes in the medium or imperfect operation of the head motor and medium transport systems.
When recording and reproducing wideband signals containing data and synchronizing information, such as picture or video data information and periodically occurring timing or pilot tone synchronizing information, particularly with rotary head video tape recorders, time base errors often occur in each period of the reproduced wideband signal. A color television signal is a wideband signal including periodically occurring vertical and horizontal pulses and pilot or color burst synchronizing information and video data information. Such signals are particularly sensitive to timing errors. One form of time base error encountered is commonly referred to as velocity error. Such errors arise from geometric tape guiding inaccuracies that cause differences between the relative head to tape velocity during recording and reproduction. These errors result in a progressive phase shift of the color subcarrier during a horizontal line. A display of a color television signal with such phase shifts will show a progressively cumulative change in color (hue) toward the right side of the video display. This invention is particularly suited to compensating such progressive time base errors which occur during a period of a time varying signal.
Head to tape velocity errors comprise one component of general time base error. Other components of time base error are the line by line mispositioning of the horizontal synchronizing pulses (H sync) and the erroneous time or phase relationships between components of the synchronizing information, such as H sync and color burst. Line by line time base errors can be compensated by correcting the phase of the reproduced television signal line by line in accordance with the detected position error of each horizontal synchronizing pulse and the detected phase error of each color burst. A known system of correcting such time base errors utilizes adjustable time delay devices inserted in the signal path. In such systems, the time base error is measured and the amount of time delay inserted in the signal path is adjusted to compensate for the measured error. One known system disclosed in U.S. Pat. No. 3,202,769 utilizes a voltage variable delay line in the signal path. A voltage corresponding to the measured time base error is applied to the delay line to provide the necessary delay for the analog television signal. Other known systems utilize lumped element and glass delay lines to compensate the analog color television signal as it is transmitted along the signal path.
More recently, the utilization of digital techniques has been proposed to provide the necessary time base correction. One such system is disclosed in commonly assigned U.S. Pat. Application Ser. No. 464,269, filed Apr. 25, 1974, by Maurice G. Lemoine for a TIME BASE CORRECTOR now abandoned in favor of continuation Ser. No. 557,708. In this digital system, each line of the analog television signal is converted to a digital form. The digitized line is then stored in a controllable digital delay device. The delay is determined by the error signal to provide the necessary time base correction.
Although the systems disclosed in U.S. Pat. No. 3,202,769 and U.S. Pat. Application Ser. No. 557,708 correct line by line time base errors at the beginning of each color television line, they do not eliminate the effects of errors progressively cumulating within each television line or period of a signal.
Velocity error compensation has heretofore been known. U.S. Pat. No. 3,428,745 is one example of known velocity error compensators used in conjunction with an analog delay line time base error corrector. The time base error corrector compensates for line by line errors at the start of each television line. The velocity compensator further provides a continuous phase compensation of each color television line to adjust chroma phase for purposes of eliminating the progressive intra line time base error that occurs during a line to cause an undesirable shift of hue progressing from left to right in the television picture. Such velocity error compensators approximate the progressively cumulative error of a line by deriving an error compensation signal from comparisons of color burst phase information of widely separated lines (64 lines in quadruplex video tape recorder) of the television signal. This approximation technique does not provide an error compensation signal related to the actual cumulative error of the line whose time base is to be adjusted. Furthermore, such compensators are unsuitable for use in conjunction with digitized time base error correctors such as disclosed in U.S. Ser. No. 557,708. Existing velocity compensators, moreover, vary the chroma phase by using the same vernier delay line utilized for time base correction. The resultant disadvantage is that the amount of velocity compensation range is limited to the residual delay range left in the delay line after time base correction. This range may be less than that required for complete chroma phase compensation, which can amount to .+-. 180.degree. of phase shift from the beginning to the end of the television line.
Some time base error compensators rely on averaging techniques to compensate for progressively cumulative time base errors. Generally, such devices provide an error compensation signal dependent upon the average of several periods of accumulated error. Such devices are incapable of rapidly responding to high rate and, especially, large line to line time base error changes.
Line by line analog delay line time base compensators have been employed in color television signal processing systems to compensate progressively cumulative errors in each line of the television signal. Such existing systems employ voltage controlled adjustable delay lines to delay the analog color burst signal of one line so that it can be phase compared to the analog color burst of the next succeeding line. This requires a delay line capable of providing a precise one line delay. Such adjustable delay lines are very expensive and require careful control of the electrical length to make certain that the delayed analog color burst is available at the correct time of the analog color burst of the next succeeding line. An analog line by line time base compensator of this kind is employed in the video tape recorder manufactured by Ampex Corporation under the model designation AVR-1.